Electron emitter



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May '19, 1953 Filed March 31, 1950 J. M. LAFFERTY ELECTRON EMITTER 2 Sheets-Sheet 1 Ceb; La 5;

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Inventor: James M. Laffer'tg. 1 M 13M His Actorne g.

' ELECTRON EMITTER Filed March 31, 1950 Fig. 2.

J. M. LAFFERTY 2 Sheets-Sheet ,2

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Inventor: James M. Laff'ert sq l-li s Attorngg.

Patented May 19, 1953 James M. Lafierty, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York.

. Application Mal-ch31, 1950, Serial No. 153,212

8 Claims. (01. 313-345) My invention relates to improved electron emitters and particularly to metallic boride emitters.

It is generally recognized that known electron emitting materials, such as those commonly used in electric discharge devices for example, are subject to various disadvantages which limit their effectiveness as emitters or require special care in handling or storing or require special processing for rendering them effective emitters.-

As a result of an intensive search fo'r'improved electron emitting materials, I have discovered that borides of certain metals provide emitters having very desirable properties which are in general superior to those of known emitters.

I have found that the rare earth metals form borides of the general formula MeBe where Me is any metal of the rare earth group and that these borides form the essential ingredient of emitters having superior'properties: Certain of the alkaline earth metals, such as barium, calcium and strontium also form borides of this general formula and possess the desirable properties of the rare earth metal borides, although to .a lesser degree. Thorium borides of theformula THIB4 and THBG and the uranium borides UB4+UB2 (a mixture of borides) also possess desirable characteristics as the emitters.

The borides of lanthanum, cerium, and'neodymium have been used separately and work very satisfactorily. Since many of the rare earth metals, however, are not available individually and it is a matter of considerable commercial importance that mischmetal (which is a natural mixture of the rare earth metals) provides a. very satisfactory metal boride emitter. It has also been found that a mixture of borides including one selected from the rare earth borides and one selected from the group consisting of the borides of barium, strontium, and calcium, particularly barium, is particularly advantageous.

' The borides mentioned above are chemically and electrically stable, have high melting points, are generally metallic in character, and, therefore, have relatively good electrical conductivity. As a result of these qualities, electron emitters consisting essentially of these borides may be used in 'demountable systems which are frequently let down to the atmosphere without deterioration of the cathode material and no special care is're quired in storing. They are generallychemically stable so that they are not adversely affected by moisture, oxygen or even hydrochloric acid. They also have the desirable property that no activation is required. to obtain emission and emission is uniform over the entire surface and remains substantially constant with time. Since 1 the boride materials and their evaporation products are relatively good conductors, any contamination of other electrodes by deposition of the film of the evaporated material does not charge-up and as a result'cause sparking between the parts of the tube in which they are employed. thus rendering cathodes of these materials suitable for high voltage applications. These materials also are refractory and have a low vapor pressure at relatively high operating temperatures. This quality, together with the relatively good electrical conductivity which permits thick coatings to be applied, provides emitters having very long life. As will be discussed in further detail, the materials'also emit currents of relatively high density and may be operated at high temperatures. -Oathodes employing these materials are also capable of either direct or indirect heating and may be easily fabricated since they may be applied as a spray or by dipping or painting. They may also be pressed and sintered to providea solid emitter of the boride material.

' i In the accompanying drawing, Fig. 1 illustrates the characteristics of a number of emitters embodying the present invention; Fig. 2 illustrates a cathode embodying the invention; Fig. 3 is an enlarged partial elevational view in section of the cathode of Fig. 2; and Figs. 4, 5 and 6 illustrate modified cathode constructions als'oembodying the present invention. 'A- comparison of a number of borides of the present invention with some well-known emitters is shown in Fig. l of the drawing in which the logarithm to the base In of the ratio of current to absolute temperature squared is plotted as ordinates and the ratio of 1000 to absolute temperatureis plotted as abscissae. These quanti-' ties are derived from data taken on emission current as a function of temperature and are chosen. forthe purpose of the plot since the resulting curves are'significant with respect to Dushmans where In Fig. l the slope'of the variousecuwes corre: sponds to the work function of the material and the intercept on the Y-axis corresponds to the;

logarithm cf the constant A in Dushmanfs eg uation. It will be noted that emission current mcreases with increasing temperatures and-thatthe.

materials may be employed for: relatively .2; sh.

The value of .7 for corresponds to an absolute temperature of 1428.5 degrees Kelvin.

fiheborides are relatively easy to produce and may to, :adizantagerbe .formed by reacting the metal with amorphous boron, pcwder by :heating in [vacuum 1G1. in an atmosphere .of: hydrogen or inert. gas. SllQh 8 S'I=h li -m:. l3 ars n at a t mperaturelof 1137556; to 1850? c. depending-upon the particular-metal involved. The amorphous heron p wder-and the inetaltpowder or metal filmesarc m xed t ether and tweed prior t the heating. I-he l 1t of this operation is. a eintered ma swh c ma be pul ze in abal} m an thenmadeintc a spray 0-1. paste av-mixing w h a su t b bind r-or carrier su h mr ace a e .01" a: ellulo bi de A n a e ezi at v the powdeiama b pr ssed into a esired shap and sintered at a tempratu-re of np cx metely 1 17 C. o hi h r u wy melti P nt 9 th qrid in e o ro.- de a s lid bcr b ds itable shap for use as an emitter. I

In 22 v cf th -d w ng. y. ?e? hQWn. a athoden trwt cnem l i eetl -w ns-- A v und: une t n wir s formed intoa helix; over a core wire 2- andthe outer surface of the .aasegnbly is ceated with a boride 3 i hma-r b ap lie e Pas ewd r d bcrid an a uitab volatilev ca i i s sw a ens -e1 acetat A t r th -ben he n applied. t e a s mbl is heated in va um t a temperature inthe neighborhood of 1600 C. t :s 'n s and e rees- V n tsbcw n Fi 2 the flethqs e-mer be suported .i.-. r 1a pa -Qt ead co du to s land 5, one end being connected directly tqconduc ten 4 :the. other, connect-ed to conductor t rbyr silien cqndu t r'lh cathod i heated by th Passa cf cu r h u h t e supporting wire I.

n 4 and. 5,1v ha e s wn xem s. of a ca hede struc u m loyin sc id oride d e a ca hod n E a Y th li d 'iq rod ofboridelisj medas previously. described and. supported within terminal caps 8 and 9 whichare pressed onto the endsof the boride od, T F-he ter inelsa c nect d. wi h-sui ab e support. and .lGElCl-vllfl wires m. and l i which may be sealed through the envelope of an electric discharge device in a manner well understood 'in the art. As sh'cwnl-in.,4, the rod 1 is indirectly heated by ga heater "coil I2 surroundtemperatures.

I 4 ing the rod and supported by lead-in conductor I 2. In Fig. 5, the boride rod 7 is arranged to be directly heated by passage of current through the rod. The rod is supported by leadin conductors l3 and M which surround the end of the rod 1 and over which a suitable conductive coating I5 is applied to improve the contact between the lead-in conductors and the rod.

In Fig. 6, I have shown in modified form a cathode applied to a planar electrode type of ,deyice -in which the electrodes are shown as an anode $6, a screen grid H, a control grid l8, a cathode l9, and a heater element 20. As (illustrated, the cathode is in the form of a disk haying. a rim 2i defining an open end recess on the side of the cathode facing the anode. Ellis reeegg is filled .with a body of boride materlal :22 w may be prepared by starting with the boride. powder and a suitable carrier and and filllll 'illlllg as previously described. In theoperation of the device described in Fig. 6, the-cathode may be heated by radiation from the heater element 29 and in addition by electron rdm nt by keecine the he 1 re en 0 t e nct ntia o 0d e the {mesa-ic d cended t i a par-v nttha he lac-r d mit en c t e P ent i ve ti ema wbe ed: 1 1 a 0, a mb at t r i he essentia c mme t f lin s? 'Qei Q- s messe le eseli lew el it m h B descr b in he sa n h M l et-cheese be me -y u thou e a ll such -nrod fie ti9ns a l Within th rue irit acd c eq mi? vent o when lqla m endr n e y tettens fei ntc th Un ed S a el A t i bi ttb eei elu l ne m ttfi c ns tin esse iall t lea -onemeta' b d ecte mm h s-rcu mee in o fi s qr cal i m bariu str tium tho di n an there na th. e a a f t p rt s said emitter and-provid ng a c cnduc t iveconnectienthete ith A th micnic cath de inc d n a t itt comprising as the ess' tial ingredient, atleast selectedfrcm the group consistmetal hex-aha a mitter at "m tels therewith.

References Cited in the flle of this patent UNITED STATES PATENTS Number Name Date 1,023,485 Thowless Apr. 16, 1912 2,111,636 Tjoflat May 17, 1938 2,501,089 Pomerantz Mar. 21, 1950 2,502,331 Malter Mar. 28, 1950 OTHER REFERENCES Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 5, pages 23-32, 1934, Longman and Green 8; Co. 

8. A CATHODE COMPRISING A CONDUCTIVE SUPPORT PROVIDING A HEATER AND A COATING COMPRISING AS THE ESSENTIAL INGREDIENT THEREOF AT LEAST ONE METALAL BORIDE, THE BORIDES BEING SELECTED FROM THE GROUP CONSISTING OF THE BORIDES OF BARIUM, STRONTIUM, CALCIUM, THORIUM, URANIUM AND THE RARE EARTH METALS. 